1. Field of the invention
Fertilizer granules or prills which have been coated with sulfur to slow the rate of dissolution of the nutrients in the soil when they are applied as a fertilizer can be coated with a sealant to improve the slow release properties of the particles. This invention relates to sealant compositions with improved slow release properties and abrasion resistance. Small particle inorganic materials are added as conditioners to provide acceptable flow properties.
2. Description of the prior art
The process for sulfur coating of urea granules or prills was developed in 1968 by the Tennessee Valley Authority (TVA) in Muscle Shoals, Alabama, as an economical system for reducing the rate of dissolution of urea particles when they are applied to the soil as fertilizer. U.S. Pat. No. 3,342,577 describes this sulfur coating process and also the sealant material that was necessary to fill in the holes that result naturally in the sulfur coating as it cools. The TVA process is also described in Sulphur Inst. J. 4(3) 2-6 (1968), ibid. 8(4) 2-5 (1972a) and ibid. 8(4) 6-8 (1972b). The TVA recommendation for the sealant is a mixture of 30% polyethylene resin in 70% bright stock mineral oil. A conditioning agent is applied over the sealant to provide free flowing particles.
U.S. Pat. No. 4,042,366 describes a modification of this process in which particles of water soluble nutrients are embedded in a soft petroleum wax coating to provide anticaking properties to the coated particles as well as nutrient benefits.
The process is further described in Sulfur Coating of Urea Treated with Attapulgite Clay, Gullett, G. L.; Simmons, C. L.; and Lee, R. G.: presented at the 198th American Chemical Society meeting in Miami Beach, Fla., in September 1989.
The requirement for a sealant for sulfur coated urea (SCU) has been documented by McClellan and Scheib (Sulphur inst. J. 9(3/4) 8-12 (1973)), and by Scheib and McClellan ibid. 12(1) 2-5 (1976).
A description of slow release urea and NPK fertilizers is given in Hort. Rev. 1 79-140 (1979).
A number of products have been developed recently for the purpose of providing sealed SCU compositions that have improved slow release properties and also do not require the use of a conditioner. U.S. Pat. No. 5,300,135 describes the use of mixtures of polymers and hard waxes as sealants for SCU. SCU sealed with these compositions are free flowing particles which do not need a conditioner. The addition of the polymer provides improved abrasion resistance over SCU sealed with the wax alone. U.S. Pat. No. 5,423,897 describes surfactant modified hard waxes used as sealants for SCU. The surfactants provide anti-blocking properties in the waxes, and the coated SCU does not require a conditioner. U.S. Pat. No. 5,478,375 describes sealants for SCU which are mixtures of fatty esters of polyhydric alcohols and polymers, crosslinked with a transesterification catalyst. These compositions do not require a conditioner. U.S. Pat. No. 5,599,374 describes a urethane polymer coating for SCU which produces free flowing particles with improved impact resistance.
The use of hard non-blocking sealants, while it improves the slow release properties and eliminates the need to apply conditioners, does not provide adequate impact resistance. Product produced by these methods must be transported, blended and applied by methods that avoid high impact or vigorous abrasion, or a significant portion of the slow release properties may be lost. Urea and blended fertilizers, when they are mixed and bagged, must be labeled with their water insoluble nitrogen (WIN) content, which is a measure of the nitrogen which is not immediately soluble in water, and it is the responsibility of the manufacturer to assure that the value does not decrease below the labeled value during shipping and storage. Thus a significant loss in WIN resulting from handling in modem high-speed equipment can create a situation in which the fertilizer is mislabeled and subject to recall.